1. Field of the Invention
The present invention relates to a method of controlling a liquid crystal display device. More particularly, the present invention relates to a method for eliminating deficient image on a liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices are widely adopted to display information in recent years. With the rapid advance in semiconductor fabrication technologies, liquid crystal display devices having the advantages of low power consumption, streamlined body, high resolution, high color saturation level and long lifetime are widely used in many types of electronic products such as the liquid crystal display screens of notebook computers or desktop computers as well as liquid crystal display (LCD) televisions.
FIG. 1 is a circuit diagram of a conventional liquid crystal display panel. As shown in FIG. 1, the display panel circuit includes a plurality of data lines 100, a plurality of gate lines 102, a plurality of thin film transistors 104, a plurality of storage capacitor 106 and a plurality of pixel capacitors 108. One of the terminals of each storage capacitor 106 is coupled to the gate line 102 and the other terminal of the storage capacitor 106 is coupled to the source of the thin film transistor 104. One of the terminals of each pixel capacitor 108 is coupled to the source of the thin film transistor 104 and the other terminal of the pixel capacitor 108 is coupled to a common potential 110, which generally is a predetermined direct current or alternating current potential.
To display an image, the gate driver will provide a positive scan voltage such as +12.5V to the selected gate line 102 so that the transistor is being on. Then, the storage capacitor 106 and the pixel capacitor 108 will receive the gray-scale potential from the data line 100 so that the gray-scale potential received by the pixel capacitor 108 and the common potential 110 generate a gray-scale voltage difference that biased rotation in the liquid crystal. On the other hand, the gate driver will provide a negative scan voltage such as −12.5V to all the unselected gate lines 102 so that the transistors are cut off. Because the pixel capacitor 108 is small, the storage capacitor 106 is used to maintain the gray-scale potential until the next gray-scale potential input arrives.
FIG. 2 is a diagram showing the timing sequence of various signals during the shutdown period of a conventional liquid crystal display device. As shown in FIGS. 1 and 2, the liquid crystal display panel is assumed to be a normally white LCD. When a power-off instruction is transmitted, the power indication signal will change from a state of high logic potential to a state of low logic potential. Then, the source driver will output a black frame such as 00H. After the passage of at least a vertical synchronous timing period (that is, at least a frame period), the source driver will output a white frame such as 3FH. The white frame is used to discharge the voltage between the terminals of each pixel capacitor until the potential difference reaches a near zero value. Finally, the positive scan voltage control signal VGH and the negative scan voltage control signal VGL are simultaneously disabled to shut down the positive scan voltage source and the negative scan voltage source.
In the foregoing shutdown sequence, the presence of a parasitic capacitor between the source and the gate of the thin film transistor leads to a coupling between the scan voltage and the storage capacitor 106. Hence, the voltage in the storage capacitor 106 will fluctuate when the positive scan voltage source and the negative scan voltage source are simultaneously shut down. The voltage fluctuation often leads to a biased rotation in the liquid crystal molecules and results in the production of a deficient image. Furthermore, with the simultaneous shut down of the negative scan voltage source and the positive scan voltage source, the voltage variation on the gate line 102 will also affect the voltage difference between the terminals of the pixel capacitor. Consequently, there will be an inversion of the liquid crystal molecules resulting in the generation of a deficient image. The appearance of a deficient image on a high-quality display product such as a liquid crystal display device is often not acceptable.